Wave division multiplexing (WDM) optical networks are well known in the art. A WDM channel is typically transmitted by a single mode semiconductor laser. Information to be communicated is imposed on the light by modulating the laser current or by externally modulating the light by applying a voltage to a modulator coupled to the laser source. A receiver employs a photo-detector that converts the light into electric current. Typically, there are two employed methodologies for detecting the received light: direct detection and coherent detection.
Optical transmission systems impose data on the amplitude of an optical signal, where the light is switched between on and off states (i.e., “1” and “0”). In direct detection, a photo-detector receives the modulated optical signal and converts the same to an electrical signal representative of the optical power. This is typically amplified and communicated to a decision circuit that compares the signal to a reference value and outputs an unambiguous “1” or “0.” One type of modulation format that encodes the data on the phase of the optical signal is known as optical differential phase keying (ODPSK). In this instance, since photo-detection does not respond to changes in phase of the incident light, a discriminator is utilized prior to the photo-detector to convert the phase changes into power values that the photo-detector can detect.
A photo-detector cannot distinguish between individual wavelengths. Therefore WDM systems employ passive filtering to separate the wavelength channels at the receiver such that each detector sees a respective channel. This consequently limits channel spacing.
Coherent detection treats optical signals in a manner analogous to RF, with response to the amplitude and phase of each wavelength. In coherent detection, an incoming optical signal is mixed with light from a local oscillator source. When the combined signals are detected by a photo-detector, the photocurrent contains a component at a frequency that is the difference between the signal frequency and the local oscillator frequency. This difference is known as an intermediate frequency and contains all the information (amplitude and phase) carried by the optical signal. Since the new carrier frequency is much lower, all information can be recovered using typical radio demodulation methodology. Coherent receivers only see signals that are close in wavelength to the local oscillator and thus by tuning the local oscillator's wavelength, a coherent receiver operates in a manner analogous to a tunable filter. Homodyne detection produces a photocurrent that is passed to a decision circuit that outputs the unambiguous “1” or “0” values. Heterodyne detection requires that the photocurrent be processed by a demodulator to recover the information from the intermediate frequency. Balanced detection replaces a 2:1 combiner with a 2:2 combiner, where each of the outputs are detected and the difference then taken by a subtracting component.
FIGS. 1 and 2 are high-level schematics of balanced photo-detection and single-ended photo-detection, respectively. In FIG. 1, a balanced photo-detection system 100 receives an incoming optical signal applied to a polarization beam splitter 102. The x-polarization is applied to a 1×4 90° phase and polarization hybrid 104a and the y-polarization to 1×4 90° phase and polarization hybrid 104b. A local oscillator (LO) 106 is coupled to a polarization beam splitter 108 such that the x-polarization is applied to hybrid 104a and the y-polarization to hybrid 104b. Each hybrid 104a, 104b has four outputs with four respective polarization states. These are received by a plurality of photo-detectors 110a and 110b, respectively, which output a corresponding photocurrent. The respective signals are 180° out of phase from each other and combined at 112a, 112b and sampled by four analog-to-digital (A/D) converters 114a, 114b, respectively. The sample values are processed by a digital signal processor (DSP) 116 to calculate the complex envelope of the signal electric field over time.
FIG. 2 illustrates a single-ended photo-detection system 200 where an incoming optical signal is applied to a polarization beam splitter 202. The x-polarization is applied to a 1×2 90° phase and polarization hybrid 204a and the y-polarization to 1×2 90° phase and polarization hybrid 204b. A LO 206 is coupled to a polarization beam splitter 208 such that the x-polarization is applied to hybrid 204a and the y-polarization to hybrid 204b. Each hybrid 204a, 204b has two outputs with two respective polarization states. The top two outputs have the local oscillator in one state of polarization (horizontal) and the lower two outputs have the local oscillator in the orthogonal state of polarization. These are received by a plurality of photo-detectors 210a and 210b, respectively, which output a corresponding photocurrent. The respective signals are 90° out of phase from each other and sampled by four analog-to-digital (A/D) converters 214a, 214b, respectively. The sample values are processed by a digital signal processor (DSP) 216.
In the expedient utilizing single-ended photo-detection such as shown in FIG. 2, only one output of the combiner is employed. Digital signal processing (DSP) based coherent optical communication provides significant performance advantages with respect to linear distortion. The aim of digital coherent detection technology is to extract both the amplitude and phase information of a modulated optical signal so that a linear digital finite impulse response (FIR) equalizer can be used to perform chromatic dispersion (CD) compensation, polarization recovery, and polarization mode dispersion (PMD) compensation in the electrical domain. As described above, typically, the optical field of the modulated signal is extracted by coherent mixing continuous wave light from a local oscillator prior to photo detection. Note that the coherent-mixed term is linearly proportional to the optical field of the original signal, but the direct square-law detection of the modulated signal will cause distortion to the extracted signal. Such distortion may severely degrade the performance of DSP-based dispersion compensation, polarization recovery and PMD compensation.
One drawback of single-ended photo-detection is the relatively high local-oscillator-to-signal power ratio (LOSPR), which can be on the order of 18 dB or more. Such a high LOSPR imposes serious constraints on receiver design. A high local oscillator power at the receiver can introduce at least a 9 dB loss by the polarization-diversity 90° hybrid. A high LOSPR also puts a more stringent requirement on the LO RIN (relative intensity noise) specification.
Accordingly, there is need in the art for a better solution to improve the performance of a digital coherent receiver using single-ended photo-detection.